A typical cellular radio system, such as the CDMA system (Code Division Multiple Access) comprises a set of subscriber terminal equipments and base stations. When a subscriber terminal equipment does not have a connection on a normal traffic channel, it monitors the paging channel of a base station and when necessary, sends messages on an access channel to the base station. When a connection is established between the subscriber terminal equipment and the base station in a cellular radio system, either the subscriber terminal equipment or the base station sends a connection establishment request. In the CDMA cellular radio system, a specific Random Access Channel (RACH) is used as an access channel on which the subscriber terminal equipment initiates call establishment. The base station responds to the messages of the access channel on an Access Grant Channel (AGCH). In this case one phase is selected from among all the available phases of a pseudonoise code for a random access type connection. When a traffic channel connection is to be established or when responding to a connection establishment request of the base station, the terminal equipments send via the access channel a connection establishment message to the base station which forwards it to other parts of the system, typically to a base station controller where traffic channel resources are allocated to the connection.
In the CDMA system, each base station sends continuously a pilot signal. If the base station has several sectors or beams, each sector or beam of the base station has typically a specific pilot signal. It is previously known to use a pilot signal for identifying a base station, a beam or sector of the base station. A pilot signal is a spread-coded signal without data modulation which signal each base station sends continuously to its coverage area. The terminal equipment may identify the base station transceiver units by the pilot signal because the phases of the spreading codes in the pilot signal differ from one another. The subscriber terminal equipments carry out measurings of pilot signals continuously.
In a typical CDMA system using direct sequence, the transmitter and the receiver have to be synchronized so that the sequence of the received signal and the sequence of the local oscillator are in the same phase. Continuous code channels, such as the pilot channel, can be used for synchronization in the direction of transmission from the base station to the subscriber terminal equipment. The subscriber terminal equipment can search for the code phase and thus synchronize to the transmission of the base station. In the opposite direction of transmission from the subscriber terminal equipment to the base station, the subscriber terminal equipment starts the transmission and the base station searches for the code phase. In the direction of transmission from the subscriber terminal equipment to the base station a problem is caused by the distance of the subscriber terminal equipment from the base station, that is, by a near-far problem. In that case, the subscriber terminal equipment has to estimate the suitable transmission power. The base station cannot, however, control the transmission power of the subscriber terminal equipment until it has synchronized itself to the correct code phase.
In addition to a circuit-switched connection, the base station and the subscriber terminal equipment can communicate also on a packet-mode connection which is used in the PRMA system (Packet Reservation Multiple Access). In that case the channel needed for sending the signal is only reserved for the duration of the signal at that moment and it is released immediately when the signal is discontinuous or when it ends or has a break. If the signal is sent discontinuously, the connection is synchronized and the channel is reserved again after each break. In a packet-mode transmission, a transfer from one carrier frequency to another and the re-establishment of a connection require a fast access process.
The prior art access process, which typically lasts hundreds of milliseconds, restricts by its slowness the performance of the radio system and strains the resources. Furthermore, as the establishment of a connection takes a long time and the transmission power of the subscriber terminal equipment cannot be controlled during that time, the subscriber terminal equipment causes interferences in the radio system.